Audiometric testing of hearing abilities usually considers two different aspects. The first aspect is to determine the degree of hearing loss throughout the audible frequency spectrum by a test procedure using pure tone frequencies within a given frequency range. For this purpose, a stepped sequence of pure tone frequencies is presented to each of an examinee's ears through earphones, the intensity level at each of said tone freuqencies being varied up and down several times to accurately ascertain the examinee's hearing threshold for each of said tone frequencies.
The second aspect relates to the testing of the ability to discriminate speech signals. For this purpose, selected discrete words of speech are presented to the examinee's ears with varying intensities.
A further aspect is the testing of the examinee's hearing abilities in the presence of background noise. For this purpose, test signals (audio tone signals or speech signals) are individually presented to one of the examinee's ears whilst simultaneously presenting a masking noise to the other ear.
Prior art audimetric systems use two different kinds of apparatus for performing tone frequency testing, on the one hand, and speech discrimination testing, on the other hand.
Such systems for performing tone frequency audiometric testing usually comprise a sine wave generator and means for varying the frequency in predetermined steps and means for varying the output intensity. Prior art apparatus for speech discrimination audiometric testing commonly comprise a magnetic tape deck or a disc player for reproducing test words recorded on a magnetic tape or on a disc. Another known type of audiometric apparatus for reproducing speech signals uses a rotating drum on which the signals are recorded in adjacent tracks.
It is a first object of the invention to provide an audiometric system using only one single apparatus for both reproducing pure tone audio frequency signals for hearing threshold testing and speech signals for speech discrimination testing.
Most of said prior art audiometric apparatus for performing hearing threshold testing using pure tone frequency signals are operated automatically following a given operating program. In such automatic apparatus, a predetermined sequence of stepped tone frequencies is produced, and the apparatus includes a potentiometer which is driven by a reversible motor for intensity variation. At any of said frequencies, the amplitude level is continuously increased until the examinee hears the signal. As long as the examinee can hear the signal, he pushes a button causing the amplitude level to decrease again until the signal fades from audibility. Upon release of the button, the signal fades into audibility again. This cycle is repeated several times at any given frequency to ascertain the examinee's hearing threshold. Simultaneously, the entire test sequence is recorded by a synchronously coupled automatic recorder.
However, while such automatic audiometer apparatus has proved satisfactory for routine testings, for example in industrial medicine where employees who must work in noisy environments have to undergo frequent and regularly scheduled hearing tests to prevent ear damage, such automatic audiometer apparatus has not proved efficient in performing thorough audiologic testing as a basis for accurate individual matching of a hearing aid.
Efficient audiologic testing requires operating facilities enabling the examiner to individually select or repeat appropriate test signals, on the one hand, and requires most simple operation of the apparatus, on the other hand, to enable the examiner to pay his full attention to the examinee to observe his reactions and also to take into account such circumstances as the examinee's age, intelligence, concentration and tiredness.
A second concern is to provide an audiometer system which is most simple to operate and allows for individually selected sequences of test signals.
While some tone audiometers are known having manually operable control means for varying the frequency and the amplitude level of the test signals by the operator, free selection of any available test word is very difficult in prior art speech audiometers of the above-mentioned types using a magnetic tape or a disc as a storage medium. It is very difficult to find a special test word recorded anywhere on a magnetic tape within a reasonable time even when a band counter is used, and even the simple repetition of a special test word involves the problem that the tape cannot be rewound exactly enough to prevent that the word will be repeated only in part or together with the end of the preceeding word. The same applies when using a disc player. While it is easier to find a special test word in a speech audiometer using a rotating drum on which the individual test words are recorded in separate adjacent tracks, such type of audiometer suffers from the disadvantage that it is very bulky.
Thus a further concern is to provide an audiometer system which includes audiometric speech discrimination testing, allowing for quick and exact selection of any one of a number of available test words recorded on a storage medium.
Further problems of prior art audiometers referred to above include problems of noise, wear of the magnetic tape or of the phonograph record, signal distortion and non-linearity. Especially in speech discrimination testing, background noise is a major problem causing a high noise to signal ratio specifically at the low intensity levels required for such testing.
Moreover, measurements have shown that many existing audiometers suffer from imperfect operation due to harmonic and other distortions.
Therefore, the audiometer system must generate high quality audiologic test signals with minimum background noise and minimum signal distortion.